1. Field of the Invention
The present invention relates to a side light type spread illuminating apparatus, and more particularly to a spread illuminating apparatus which is used as a lighting means of a liquid crystal display device and in which a hot spot attributable to light coming from a light emitting diode as a light source is inhibited.
2. Description of the Related Art
A backlight system which irradiates a liquid crystal display (LCD) area from its behind has been conventionally used as a spread illuminating apparatus (a lighting means) for an LCD device. Such the spread illuminating apparatus (backlight system) includes a light conductor plate having light transmitting property, and a plurality of light emitting diodes (LEDs) or a cold cathode lamp as a light source disposed at a side surface of the light conductor plate, wherein light, which is emitted from the LED or the cold cathode lamp and introduced into the light conductor plate through the side surface (light inlet surface), exits from a light outlet surface of the light conductor plate while traveling inside the light conductor plate with repeated reflection and diffusion therein. In order to allow the light introduced in the light conductor plate to efficiently exit from the light outlet surface, a light reflector plate is provided at a major surface of the light conductor plate opposite to the light outlet surface.
The spread illuminating apparatus structured as described above, however, fails to achieve a favorable display quality, and a spread illuminating apparatus is disclosed as shown in FIG. 7 which, in order to prevent emission lines from occurring at the LCD area and brightness from lowering, is structured to include at a light outlet surface of a light conductor plate 104 a light absorber layer 108 disposed along a light source lamp 102 having a reflector 103 disposed at its behind and sandwiched between a light diffuser sheet 105 and a light reflector lay 109, and also to include another light absorber layer 108 which is printed on a major surface of the light conductor plate 104 opposite to the light outlet surface, is disposed along the light source lamp 102 and which is covered by a light reflector sheet 107, wherein light emitted from the light source lamp 102 is adapted to pass through a prism sheet 106 disposed on the light diffuser sheet 105 when exiting the light conductor plate 104 (refer to, for example, Japanese Patent Application Laid-Open No. H10-161119: Paragraphs 0020 to 0025, FIG. 1).
Also, the above-described spread illuminating apparatus, when incorporating LEDs as a light source, causes a non-uniform brightness that occurs on the light outlet surface of the light conductor plate between the area in front of each LED (high brightness) and the other area (low brightness), for example, the area between two adjacent LEDs. FIG. 8 shows a relevant portion of a spread illuminating apparatus which, in order to prevent such the non-uniform brightness defect, includes a light reflecting and absorbing sheet 7 disposed along a light inlet surface of a light conductor plate 2 so as to cover LEDs 5 mounted on a printed board 6, and also includes a light absorbing surface 8 formed on the inner side of the light reflecting and absorbing sheet 7 thus covering the LEDs 5. In the spread illuminating apparatus shown in FIG. 8, lights L1 and L2 out of lights emitted from the lowest part of the LED 5 and entering the light conductor plate 2 from the light inlet surface fall incident on a light outlet surface of the light conductor plate 2 at a critical angle θ or a smaller angle and are absorbed at the light absorbing surface 8 formed at the inner side of the light reflecting and absorbing sheet 7 while a light L3 thereof falls incident on the light outlet surface of the light conductor plate 2 at an angle larger than the critical angle θ and is fully reflected at the light outlet surface. Thus, lights direct from the LED 5 are prohibited from exiting from an area located in front of the LED 5 on the light outlet surface of the light conductor plate 2, whereby brightness non-uniformity at the light outlet surface attributable to a high brightness provided at the area in front of the LED 5 can be reduced or removed (refer to, for example, Japanese Patent Application Laid-Open No. 2003-242817: Paragraphs 0045 to 0047, FIG. 3).
In the spread illuminating apparatus of FIG. 7 disclosed in the aforementioned Japanese Patent Application Laid-Open No. H10-161119, the brightness at the display portion is enhanced by providing a light absorber layer at each of the both major surfaces of the light conductor plate. And, in the spread illuminating apparatus of FIG. 8 disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-242817, a light absorbing region is provided to cover above and in front of an LED so that light emitted in front of the LED is absorbed thereby lowering the brightness in front of the LED (high brightness) thus achieving a brightness uniformity.
Recently, in the spread illuminating apparatuses, a demand for expansion of effective illumination area is more and more increased, and accordingly it is needed to take further steps to inhibit brightness non-uniformity (hot spot) in the area in front of an LED. On the other hand, a light reflector plate may be extended to cover below an LED in order to effectively utilize light emitted downward from the LED, but this can possibly cause a hot spot problem.